I would drop to a 17" wheel myself. If it it clears the front brakes why run something bigger and heavier. 18" just look too big on a hatchback for me. I think a 17x8 (48mm offset) with a 245/40/17 would probably work. Maybe some slight rub in the back with weight.
Then buy a pair of 16x8...
These devices tend to be optimistic especially with trap speed. At the drag strip your trap is an average over the last whatever feet so it's going to be lower than what these are reading.
That would need to be tested. From my 1.8T days, the more aggressive the clamp the more sensitive the car was to drive at part throttle. It got to a point to where it was undrivable under 5K rpm. The turbo was a K03 and it was really hard to control boost since it was so small. It would make...
Clamping the MAP fools the ECU into how much boost you are making. Back when I had my GTI 1.8T it was drive by wire. The ecu would start closing the throttle plate in the upper revs. The clamp fooled the ECU and would keep the TP open until redline.